The present invention relates to optical elements comprising fluorochemical surface treatments. The invention further relates to materials such as retroreflective sheetings, pavement markings and beaded projection screens comprising a binder and the surface treated optical elements. The fluorochemical surface treatment is substantially free of perfluoroalkyl groups having more than four carbon atoms. Preferably, the surface treatment is a polyfluoropolyether-based compound comprising at least one polar group or polar group-containing organic radical.
Beaded projection display screens, retroreflective sheeting used in the manufacture of roadway signs, and retroreflective paints typically include optical elements adhered through the use of a binder. In the case of beaded projection display materials, the optical elements are microscopic glass beads that act as lenses to collect projected light from the rear of the screen and focus it to relatively small spots, near the surfaces of the microspheres. The foci are approximately in the areas where the optical elements contact a front support layer. In other retroreflective materials, the optical elements act as lenses which focus the light onto a reflector (metal mirror of diffusely reflecting pigment) and once the light has been reflected off the reflector the microspheres again act as lenses to resend the light back toward the incoming light source. In order to contribute the desired retroreflective property, however, it is important that a layer of glass microspheres be present on the surface of the binder layer.
As discussed in U.S. Pat. No. 3,222,204, ordinary glass beads tend to sink into the uncured liquid binder layer. In instances wherein the individual beads are not entirely submerged, the optical properties of the bead can also be impaired by the binder wetting out the bead surface and spreading on the exposed bead surface. To address this problem, U.S. Pat. No. 3,222,204 teaches coating the glass beads with a thin surface coating of an oleophobic fluorocarbon-sizing agent. At column 5, lines 61-75, this reference states that, xe2x80x9cAqueous treating solutions of fluorocarbon chromium coordination complexes are preferred and are described in U.S. Pat. No. 2,662,835 (Dec. 15, 1953) and U.S. Pat. No. 2,809,990 (Oct. 15, 1957) and U.S. Pat. No. 2,934,450 (Apr. 26, 1960). The complex may be made by reacting chromyl chloride with a fluorocarbon monocarboxylic acid (having a highly fluorinated terminal chain or tail containing 4 to 10 carbon atoms) in an isopropanol vehicle that serves as both a solvent and reducing agent, the chromium to acid mole ratio being in the range of 2:1 to 5:1. The resultant green-colored isopropanol solution of the complex is diluted with water at the time of use. The fluorocarbon acid preferably has 6 to 8 fully fluorinated (perfluorinated) carbon atoms in the terminal fluorocarbon chain or tail.xe2x80x9d Specific working examples include chromium coordination complexes of perfluorooctanoic acid and N-ethyl-N-perfluorooctanesulfonyl glycine.
U.S. Pat. No. 4,713,295 teaches coating glass beads with a mixture of substances. The mixture comprises a first substance which if used alone would tend to make the beads hydrophobic while leaving them oleophilic and a second substance which if used alone would tend to make the beads both hydrophobic and oleophobic. xe2x80x9cFor the best results, it is preferred to use a second substance which is an anionic fluorocarbon compound, and optimally, said second substance is a fluoro-alkyl-sulphonate, for example a fluoro-alkyl-sulphonate in which the alkyl has a long chain (C14 to C18).xe2x80x9d (See Column 4, lines 8-13). The exemplified hydrophobic and oleophobic substance is a potassium fluoroalkyl-sulphonate (for example FC129 from 3M). (See column 5, lines 50-52) FC129 is a potassium fluoroctyl sulphonyl-containing compound.
The fluorocarbon surface treatment of the present invention is substantially free of perfluoroalkyl groups having more than 4 carbon atoms. The present inventors have identified various fluorocarbon derivatives that lack the previously preferred 6 to 8 carbon and longer fully fluorinated tail. Although the terminal fluorocarbon tail is considerably shorter, or absent, the surface treatments of the present invention are surprisingly effective in obtaining the desired floatation properties combined with the desired adhesion with the binder.
In one embodiment, the present invention is an optical element having a surface treatment comprising a polyfluoropolyether compound having at least one polar group or polar group-containing organic radical. The polyfluoropolyether compound has the general formula Rfxe2x80x94[X]y wherein Rf is a monovalent or divalent polyfluoropolyether group, X is a polar group or polar group-containing organic radical, and y ranges from 1 to 2. Rf comprises one or more repeating units selected from the group comprising xe2x80x94(CnF2n)xe2x80x94, xe2x80x94(CnF2nO)xe2x80x94, xe2x80x94(CF(Z))xe2x80x94, xe2x80x94(CF(Z)O)xe2x80x94, xe2x80x94(CF(Z)CnF2nO)xe2x80x94, xe2x80x94(CnF2nCF(Z)O)xe2x80x94, xe2x80x94(CF2CF(Z)O)xe2x80x94, and combinations thereof; wherein the average n value ranges from 1 to 4 and Z is a perfluoroalkyl group, an oxygen-substituted perfluoroalkyl group, a perfluoroalkoxy group, or an oxygen-substituted perfluoroalkoxy group having 1 to about 4 carbon atoms and 1 to about 4 oxygen atoms. The polar group or polar group-containing organic radical, X, is preferably selected from the group comprising carboxylic acid, sulfonic acid, phosphonic acid, carboxylate salt, sulfonate, phosphonate, phosphate esters, ammonium salts, amine, amide, alkyl amide, alkyl aryl amide (e.g. aryl amide, alkaryl amide, arylalkyl amide), imide, sulfonamide, hydroxymethyl, thiol, ester, silane (e.g. chlorosilane, silanols, siloxanes), polyoxyalkyene, cycloalkyl, heteroalkyl, organic radicals substituted with one or more of such polar groups, and mixtures thereof.
Preferred polyfluoropolyether surface treatments have the general formulas Xxe2x80x94CF2O[(CF2O)m(C2F4O)p]CF2xe2x80x94X, C3F7O(CF(CF3)CF2O)pCF(CF3)xe2x80x94X, and Xxe2x80x94CF2O(C2F4O)mCF2xe2x80x94X; wherein m and p each range from 0 to 50 with the proviso that m and p are not both 0 and X is a polar group or polar group-containing organic radical comprising CONH(CH2)3Si(OMe)3, CONH(R) wherein R is an alkyl radical having 1 to 8 carbon atoms, CONHCH2CH2Ph, CO2NH4, CH2NH2, CO2H, CH2OH, CO2CH3 and combinations thereof. Further, the polyfluoropolyether compounds having the general formula Xxe2x80x94CF2O[(CF2O)m(C2F4O)p]CF2xe2x80x94X wherein X is CONH(CH2)3Si(OMe)3; CO2H, CO2NH4 and mixtures thereof; and (C3F7O(CF(CF3)CF2O)p(CF(CF3)xe2x80x94X wherein X is CO2H, CO2NH4, and mixtures thereof, wherein m and p each range from 0 to 50 with the proviso that m and p are not both 0, have demonstrated 100% float at loading levels of about 100 ppm and less.
In another embodiment, the polyfluoropolyether compound has the general formula [Rfxe2x80x94Xxe2x80x94]nxe2x80x94Lc or [Rfxe2x80x94Xxe2x80x94Lxe2x80x94X]n wherein Rf is a monocovalent or dicovalent perfluoropolyether group, X is a dicovalent polar group or polar group-containing organic radical, n ranges from 2 to 20, Lc is a polycovalent core linkage, and L is a linkage derived from a comonomer.
The number average (Mn) molecular weight of the polyfluoropolyether compound preferably ranges from about 400 to about 10,000 g/mole. Mn is preferably greater than about 1000 g/mole and more preferably greater than about 2000 g/mole. Mn is preferably less than about 8,000 g/mole.
In another embodiment, the present invention is an optical element having a surface treatment comprising a fluorochemical compound having the general formula (CnF2n+1)xe2x80x94X wherein n is about 4 and X is a polar group or polar group-containing organic radical. The surface treatment is preferably free of heavy metals and transition metals and/or X is preferably selected from the group comprising sulfonic acids and salts thereof; sulfonamides, sulfonimides and salts thereof; amides, silanes (e.g. chlorosilane, silanols, siloxanes), and mixtures of such surface treatments.
The surface treated optical elements of the invention preferably exhibit a percent float in heptane of at least about 90%.
In another embodiment the present invention is an optical element having a surface treatment comprising a fluorochemical compound at a concentration of less than about 100 ppm based on the weight of the optical elements wherein the percent float in heptane is at least about 90%.
In another aspect, the present invention is a method of coating an optical element comprising providing an aqueous surface treatment comprising at least one fluorocarbon compound wherein said compound is free of perfluoroalkyl groups having more than four carbon atoms, coating said composition on optical elements, and drying said composition. The aqueous composition preferably comprises no more than 30 wt-% of a cosolvent with respect to the total weight of the solution.
The invention further relates to reflective articles such as pavement markings, reflective sheeting, and projection screens comprising a binder and the surface treated optical elements of the invention. The optical elements are embedded in the binder surface at a depth of about 40 to 60 percent of their diameters.